1. Field of the Invention
The present invention relates to a quadrupole mass spectrometer using a quadrupole mass filter as a mass analyzer operable to separate ions according to mass values (exactly, m/z (mass-to-charge ratio) values).
2. Description of the Background Art
A quadrupole mass spectrometer is designed to apply a voltage formed by superimposing a high-frequency (radio-frequency) voltage on a direct-current (DC) voltage, to four rod electrodes constituting a quadrupole mass filter, to allow only an ion having a mass corresponding to the applied voltage to selectively pass through the quadrupole mass filter and reach an ion detector. Recent years, a gas chromatograph/mass spectrometer (GC/MS) and a liquid chromatograph/mass spectrometer (LC/MS) produced by combining the quadrupole mass spectrometer with respective ones of a gas chromatograph and a liquid chromatograph are widely used in various fields.
A scan measurement and a selected ion monitoring (SIM) measurement are well known as a measurement mode of the quadrupole mass spectrometer. The scan measurement is configured to repetitively perform a control/processing of scanning (continuously changing) a voltage to be applied to the rod electrodes of the quadrupole mass filter, so as to scan (continuously change) a mass value for an ion to be allowed to reach to the ion detector, over a given mass range. The scan measurement shows excellent ability, particularly, in qualitative analysis for a sample containing a substance whose mass is unknown. The SIM measurement is configured to repetitively perform mass analysis for ions having ones of a plurality of mass values pre-set by a user, while sequentially changing between the plurality of mass values. The SIM measurement shows excellent ability, particularly, in quantitative analysis for a substance whose mass is known.
FIG. 6 is a schematic diagram showing a change in mass value for a target ion to be analyzed, during the scan measurement. As shown in FIG. 6, in order to allow respective target ions to pass through the quadrupole mass filter, a voltage to be applied to the rod electrodes of the quadrupole mass filter is gradually increased from a voltage value corresponding to a minimum mass value M1. Then, after the voltage reaches a voltage value corresponding to a maximum mass value M2, the voltage is rapidly returned to the voltage value corresponding to the minimum mass value M1, and a next voltage scan (mass scan) cycle will be performed in the same manner. Such a rapid voltage change inevitably causes overshoot (or undershoot) and ringing. Thus, a waiting time-period (settling time-period) is provided just after the voltage change to continue until the voltage becomes moderately stable, and, after an elapse of the settling time-period, a next voltage scan (mass scan) cycle is started to perform a substantial ion detection operation, i.e., a measurement operation.
In the SIM measurement, during a course of changing from a certain one to a different one of the plurality of mass values, the above overshoot (or undershoot) and ringing in voltage inevitably occur, as with the scan measurement. Thus, it is necessary to provide a settling time-period just after a voltage change, and, after an elapse of the settling time-period, perform a substantial ion detection (measurement) operation for the mass value corresponding to an applied voltage after the voltage change. For example, the following Patent Publication 1 includes a description that it is essential to provide a settling time-period in the SIM measurement.
In both the scan measurement and the SIM measurement, during the settling time-period, any mass analysis for components of a sample introduced from a GC or LC into an ion source is not performed. Thus, for example, in the scan measurement, as the settling time-period becomes longer, a time interval between adjacent mass scan cycles becomes larger, i.e., a duration of one mass scan cycle becomes longer, to cause deterioration in time resolution. In the SIM measurement, as the settling time-period becomes longer, a time interval between measurements for a respective one of the mass values in adjacent cycles becomes larger to cause deterioration in time resolution. Although a duration of the repetitive cycle may be shortened to enhance the time resolution, it causes a reduction in ion detection (measurement) time-period for each of the mass values, which leads to deterioration in sensitivity and SN ratio.
In a mass spectrometer disclosed in the following Patent Document 2, when a voltage to be applied to rod electrodes of a quadruple mass filter is changed in a step-like pattern to change a mass value for a target ion in a step-like pattern, a waiting time-period (settling time-period) before performing an ion detection (measurement) operation is controllably changed depending on a voltage difference between adjacent ones of the steps. This control makes it possible to reduce a total of the settling time-periods to increase a measurement time-period, as compared with a technique where each of the settling time-periods is set to a constant value assuming a maximum settling time-period. However, there remains a need for further reducing the settling time-period achievable by the conventional control, to enhance time resolution and sensitivity/SN ratio.
[Patent Document 1] JP 2000-195464A
[Patent Document 2] JP 4-289652A